Lets imagine, you search in Google Creative Common images. Found image from StarWars movie that marked as Creative Common, and you thought that you can use it in your commercial project?
If you buy image at Stock library you can always ask any required documents from marketplace that you bought this image and have right use it inside rights allowed for this image. And if something will be wrong that will be dispute between you and marketplace. And later between marketplace and author.
I don't think you can receive any documents for lawyers about any rights from SketchFab for this moment.
Ehh, i hope you understand my idea